panfrost: Determine framebuffer format bits late

panfrost: Determine framebuffer format bits late

Again, these formats are only properly known at the time of fragment job
emit. Rather than hardcoding the format, at least for MFBD we begin to
construct the format bits on-demand. This cleans up the code,
futureproofs for ES3 framebuffer formats, and should fix bugs regarding
FBO colour swizzles.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Tomeu Vizoso <tomeu.visozo@collabora.com>